Carcinoembryonic antigen (CEA), first described in 1965 by Gold and Freedman, Journal of Experimental Medicine, 121:439, is a tumor-associated antigen. CEA was characterized as a glycoprotein of approximately 200,000 molecular weight with a .beta.-electrophoretic mobility. (J. Wilson Krupey, et al, Journal of Experimental Medicine, 128:387, 1968 and J. Wilson Krupey, et al, Immunochemistry, 9:617, 1972.) Subsequent development of a radioimmunoassay (RIA) by Thomson, et al (D.M.P. Thomson, et al, Proc. Natl. Acad. Sci., U.S.A., 64:16, 1969) made it possible to detect the very low concentrations of CEA in circulating blood, other body fluids and also in normal and diseased tissues (N. Samcheck, Internal Medicine, 19:413, 1974; V. L. W. Go, et al, Cancer, 36:2346, 1975; and S. K. Khoo, et al, Int. J. Cancer, 11:681, 1973.) Two years later, Hansen, et al, (H. J. Hansen, et al, Clinical Research, 19:143, 1971) developed a modified RIA for CEA.
The results of clinical studies to date indicate that CEA, although originally thought to be specific for digestive tract cancers, is also elevated in other malignancies and in some nonmalignant disorders. (A. M. Steward, et al, Cancer, 33:1246, 1974; P. Oehr, et al, Tumor Diagnostik, 1:40, 1980; and G. Reynoso, et al, Journal of American Medical Association, 220:361, 1972.)
The experience with testing using other methods for assaying CEA has shown that CEA testing can have significant value in the monitoring of patients with diagnosed malignancies during treatment. A persistent elevation in circulating CEA following treatment is strongly indicative of occult metastatic and/or residual disease. (N. Zamcheck, Symposium of Clinical Application of CEA and Other Antigenic Markers Assays, Nice, France, October, 1977, Amsterdam, Oxford, Medica Exerpta, 1978; and E. W. Martin, et al, American Journal of Surgery, 137:167, 1979.) A persistently rising CEA value may be associated with progressive malignant disease and poor therapeutic response. (A. T. Sharin, et al, Cancer, 33:1239, 1974.) A declining CEA value is generally indicative of a favorable prognosis and good response to treatment. (J. J. Lokich, et al, Annals of Internal Medicine, 89:902, 1978.)
Patients who have low pretherapy CEA levels may later show elevations in CEA level as an indication of progressive disease.
Clinical relevance of the CEA assay is best known in the follow-up management of patients with colorectal carcinoma, but value has been shown also for serial monitoring of CEA levels in patients with adenocarcinoma arising in other digestive system organs, as well as in the lungs, breast and prostate, and in patients with epidermoid carcinomas of the lung, esophagus and genitourinary system.
The CEA level also provides prognostic information. Long-term follow-up studies of pateints with colorectal and resectable lung carcinoma suggest that the pre-operative CEA level has prognostic significance. (H. S. Wanebo, et al, New England Journal of Medicine, 299:448, 1978 and J. P. Concannon, et al, Cancer, 42:1477, 1978.)
More recently a solid phase enzyme immunoassay based on the "sandwich" principle has been developed and commercially sold by Abbott Laboratories, North Chicago, Ill., all under the tradename CEA-EIA or CEA-RIA. Beads coated with guinea pig anti-CEA are incubated with heat-treated specimen supernatants and the appropriate standards and controls. During this incubation, CEA present in the specimen is bound to the beads. Nonreactive components are removed by aspiration of fluid and washing of the beads. Goat anti-CEA conjugated with horseradish peroxidase or I.sup.125 is incubated with the beads and, if CEA were present in the specimen, the anti-CEA:horseradish peroxidase conjugate or anti-CEA I.sup.125 conjugate is bound to the CEA on the beads. Unbound conjugate is removed by aspiration and the beads washed. The beads are next incubated with enzyme substrate solution (hydrogen peroxide and orthophenylenediamine.2HCl) to develop a color which is a measure of the amount of bound anti-CEA:horseradish peroxidase conjugate. The enzyme reaction is stopped by the action of N hydrochloric acid and the intensity of color developed is read using the spectrophotometer set at 492 nm. The intensity of the color formed by the enzyme reaction is proportional to the concentration of CEA in the specimen within the working range of the assay. For the CEA-RIA assay the amount of I.sup.125 conjugate bound to the beads is determined by counting in a gamma scintillation counter. A standard curve is obtained by plotting the CEA concentration of the standards vs. the absorbance (or counts per minute). The CEA concentration of the unknowns and controls run concurrently with the standards can be determined from the cruve.
Problems with CEA assays have generally involved pretreatment and extraction techniques to liberate CEA in a biological fluid such as serum or plasma. Various heat treatment and perchloric acid extractions have been used. This invention avoids the need for elaborate pretreatment and simplifies immunoassays for CEA.